There is a panel made of a synthetic resin now as a light and inexpensive plate-like member. Such a panel has a high versatility, and thus it is used for various purposes. For example, the panel may be used as a material for a sink or bathroom shelf utilizing its high corrosion resistance, or may be used as an interior panel for a vehicle utilizing its characteristic of a lightweight.
Further, in order to structurally increase the rigidity of the panel, there has been developed a panel with an inner rib, in which a reinforcing plate (so-called inner rib) is formed in a grid in a space between a front plate and a rear plate included by the panel (See Patent Literatures 1 to 3)    Patent Literature 1: Japanese Patent Application Laid-Open Publication No. 56-120319    Patent Literature 2: Japanese Patent Application Laid-Open Publication No. 57-118965    Patent Literature 3: Japanese Patent Application Laid-Open Publication No. 10-16079